Recently, there has been a demand for the enhancement of fuel economy (fuel consumption ratio) in an internal combustion engine in view of tightening of regulations on the emission of carbonic acid gas or from the fear of the depletion of fossil fuels. To satisfy such a demand, efforts have been made to enhance fuel economy by reducing various losses in the internal combustion engine. In general, the reduction of losses can decrease an output necessary for an operation of the internal combustion engine and hence, a minimum output of the internal combustion engine can be made small. In such an internal combustion engine, it is necessary to supply fuel by controlling a fuel quantity such that even a small fuel quantity corresponding to the minimum output can be controlled.
Further, recently, a downsizing engine which acquires a required output with the use of a supercharger while miniaturizing a size thereof by reducing a displacement of an engine has been attracting attentions. In the downsizing engine, by making the displacement small, a pumping loss and a friction can be reduced so that fuel economy can be enhanced. On the other hand, while acquiring a sufficient output with the use of the supercharger, owing to an intake air cooling effect brought about by a cylinder direct injection, it is possible to prevent a compression ratio of the downsizing engine from being set low due to supercharging and hence, fuel economy can be enhanced. Particularly, in a fuel injection device used in such a downsizing engine, it is necessary to inject fuel over a wide range from a minimum injection quantity corresponding to a minimum output obtained by making the displacement small to a maximum injection quantity corresponding to a maximum output obtained by supercharging.
In general, an injection quantity of the fuel injection device is controlled based on a pulse width of an injection pulse (drive pulse) outputted from an ECU (Engine Control Unit). The longer the pulse width, the larger the injection quantity becomes, while the shorter the pulse width, the smaller the injection quantity becomes. The approximately linear relationship is established between the pulse width and the injection quantity. However, in a region where the injection pulse width is short, the injection quantity is not changed linearly with respect to the injection pulse width due to a rebound phenomenon which occurs when a movable element impinges on a stopper or the like (bound behavior of a movable element) thus giving rise to a drawback that a minimum injection quantity which the fuel injection device can control is increased. Further, there may be a case where the injection quantity does not become stable due to the above-mentioned rebound phenomenon of the movable element, and there has been a case where this unstable injection quantity causes the increase of the minimum injection quantity or causes the increase of individual irregularities among manufactured fuel injection devices.
As described above, to enhance fuel economy, it is necessary to reduce the minimum fuel quantity which the fuel injection device can control.
To reduce the minimum fuel quantity, it is necessary to suppress the bound behavior of the movable element. As a technique for satisfying such a request, in JP-A-58-214081, there is disclosed a solenoid valve drive unit where a speed of a plunger is decreased by rapidly cutting off an electric current immediately before a valve opening operation is completed (immediately before the plunger reaches a target lift amount) so that a rebound phenomenon of the plunger is suppressed whereby non-linearity of a flow rate characteristic is improved thus reducing a minimum injection quantity.
Further, as another means for reducing a minimum injection quantity, there has been known a fuel injection control device disclosed in JP-A-2009-162115. In such a fuel injection control device, an electric current is supplied to a fuel injection device from a high-voltage power source and, thereafter, the electric current is rapidly discharged so that the electric current is lowered to a first current value at which a valve element cannot be held in a valve open state or below and, thereafter, an electric current having a second current value at which the valve element can be held in the valve open state is supplied to the fuel injection device so that a delay in closing a fuel injection valve in a small pulse region can be decreased thus reducing a minimum injection quantity.